This invention relates to electric resistance heating elements, and more particularly, to plastic insulated resistance heating elements containing encapsulated resistance material.
Electric resistance heating elements composed of polymeric materials are quickly developing as a substitute for conventional or xe2x80x9cstandardxe2x80x9d metal sheathed heating elements, such as those containing a Nixe2x80x94Cr coil disposed axially through a U-shaped tubular metal sheath. Good examples of polymeric heating elements include those disclosed in Eckman et al., U.S. Pat. No. 5,586,214, issued Dec. 17, 1996; Lock et al., U.S. Pat. No. 5,521,357, issued May 28, 1996; Welsby et al., U.S. Pat. No. 4,326,121, issued Apr. 20, 1982, and J. W. Welsh, U.S. Pat. No. 3,621,566, issued Nov. 23, 1971, which are all hereby incorporated herein by reference.
Eckman et al. ""214 discloses a polymer encapsulated resistance heating element including a resistance heating member encapsulated within an integral layer of an electrically-insulating, thermally-conductive polymeric material. The disclosed heating elements are capable of generating at least about 1,000 watts for heating fluids such as water and gas.
Lock et al. ""357 discloses a heater apparatus including a resistive film formed on a substrate. The first and second electrodes are coupled to conductive leads which are electrically connected to the resistive film. The heater also includes an over molded body made of an insulating material, such as a plastic. Lock et al. ""357 further disclose that their resistive film can be applied to a substrate, such as a printed circuit board material.
Welsby et al. ""121 discloses an electric immersion heater having a planar construction which contains an electrical resistance heating wire shrouded within an integral layer of polymeric material, such as PFA or PTFE, which is wound around end portions of a rectangular frame. The frame and wound resistance wire is then secured in spaced relationship with one or more wrapped frame members, and then further protected by polymeric cover plates which allow for the free flow of fluid through the heater.
J. W. Welsh ""566 discloses a single planar resistance member having a dipped coating of thermoplastic material, such as PTFE, nylon or KEL-F, a 3M product. Welsh teaches that his element can be self-cleaning, since the heated wire is free to expand within the insulation, which is flexible.
The problems associated with metal sheathed elements in immersed fluids are generally known. These problems are caused by the industry""s need for high watt densities. High watt densities can cause high external sheath temperatures which can damage fluid and increase scale build-up, and high internal heating element temperatures which limit heater life.
The formation of hard lime scale on container walls and heating elements can be traced to the calcium carbonate (CaCO3) content of the water in combination with the scarcity of nucleation centers in ordinary water. When the concentration of the calcium carbonate exceeds its solubility, solidification often begins on the surface of the heating element. Hard lime scale begins with a few starting points on the surface of the element which attach firmly to it and extend crystals which cling to one another in a dendritic crystallization mode. This process continues as further solidification of the mineral occurs, growing layer by layer over each successive formation of dendrites. See Kronenberg, xe2x80x9cMagnetic Water Treatment De-mystifiedxe2x80x9d, Green Country Environmental Associates, LLC, Jan. 19, 2000, which is hereby incorporated by reference.
Scale produced by residential water heaters operated on hard water at approximately 160xc2x0 F. consists principally of calcium and calcium carbonate. Differences in water quality at various sites do not generally exert a strong influence on scale composition. Minor metallic constituents, such as magnesium, aluminum and iron, generally comprise less than 3% of the scale composition.
There is a slight improvement in scale resistance associated with polymer sheathed fluid heating elements; however, there remains a need in the heating element industry to improve this technology. Some of these weaknesses associated with polymer heating elements are known to include (1) the low thermal conductivity of polymeric coatings which generally prevents thick polymer coatings from being used; (2) the need to use a greater surface area to keep the polymer below its heat deflection temperature, while providing for the application""s heating requirements; (3) the high manufacturing costs associated with larger surface area heaters, and (4) the management of mechanical and creep stresses due to the differences in the coefficient of thermal expansion between metallic and polymeric materials.
The present invention provides flexible spirally shaped heating elements comprising a resistance heating material having a plurality of spiral forms distributed around a central axis, said resistance heating material containing an electrically insulating polymeric coating. This heating element has a flux or watt density which is significantly lower than that for a tubular Heating Element of substantially similar Active Element Volume (in3), but having the same or greater overall wattage rating (total watts) that the Tubular Heating Element.
In another preferred embodiment of this invention, a flexible spiral shaped heating element is provided which includes a resistance heating ribbon or wire insulated within a thermally conductive, electrically insulating polymeric coating. The resistance heating ribbon or wire is disposed into a spiral form having an external dimension sufficient to fit within a 1.0-1.5 inch opening of a standard residential hot water heater, yet provides an xe2x80x9ceffective heating surface areaxe2x80x9d (herein defined) which is at least two times greater than the effective heating surface area of a conventional metal-sheathed tubular heating element of roughly the same external dimensions.
More preferably, the spirally shaped heating elements of this invention include a surface area of about 5-60 in2/in3, and preferably about 10-30 in2/in3, which represents a great deal of improvement over Welsh ""566, which presents an effective heating surface area of only about 2 in2/in3, and Welsby et al., which presents a slightly greater surface area, but is incapable of being retrofitted within an existing 1.0-1.5 inch standard opening in a hot water heater.
Moreover, the ability for the present spirally shaped heating elements to expand and contract during heating presents a tremendous opportunity to reduce scaling of hard water deposits. The elements of the present invention are capable of developing changes in their radius of curvature, which are approximately 2-10 times greater than the minimal expansion associated with the flat ribbon of Welsh, and provide even greater expansion opportunities when compared to fixed coated wire elements, such as those described by Welsby et al, which are constrained by a frame.
The claimed heating elements, in the presence of water, can run at watt densities (or flux) of less than 20 watts per square inch, and desirably about 5-15 w/in2, with a target of about 7-12 w/in2. It is generally known that a lower watt density will reduce fluid damage and minimize scale generation.
The preferred spirally shaped heating elements of this invention can yield watt densities of less than 50%, and preferably about 10% to about 30% of the watt density of a standard Tubular Heater Element having the same Active Element Volume (in3). These heating elements minimize fluid damage, such as in the case of oil in engine block heaters or space heaters, for example, by minimizing the carbonization created by high heater surface temperatures. The elements and methods of fabrication provide a low cost heater with a minimum number of components and electrical connections.
Other improvements provided by this invention include its relatively low flux or watt density, therefore creating very low element surface and internal temperatures in immersed fluid heating applications. The polymer coatings of this invention can be provided in thicknesses of about 1-20 thousandths of an inch to provide a very low temperature differential between the resistance heating element material and the surface of the polymer coating. These flexible spirally shaped heating elements are also free to expand and contract with changes in the temperature of the heating element. This reduces mechanical stresses due to differences in the coefficient of thermal expansion between the various metallic and nonmetallic components of such heaters. The flexing also helps to break up and shed any built up scale on the heater surface. These preferred embodiments also permit nearly the entire surface area, or at least about 90-95% of the surface area of the heating element to be heated. This prevents discontinuities, or abrupt changes in the flux density of the heater surface, thereby minimizing mechanical stresses due to unheated areas in the preferred polymeric insulating coating.
The spirals of this invention, depending on the rigidity of the resistance wire, may be supported on a rod, with or without physical attachment to the rod, such as by pins, rivets or adhesive. They may be sealed or partially contained within a fluid-soluble coating or band, which dissolves quickly to permit the element to expand to its operational dimensions, which dimensions can be larger in diameter than the typical 1-1.5xe2x80x3 diameter standard water heater tank opening, or any other standard opening desired.